No bass from optical connection

[dipzy]

This is pretty strange. Maybe you could try the 3.5mm male to x2 RCA male to connect your PC via your sound card to the x3 sets of RCA connectors on the decoder?

It is very strange, the rca's won't give me the 5.1 sound i thinks

 

[Toxteth O'Grady]

 I think so too because they are 3 independently switched stereo pairs that cannot be mixed in the decoder.  i think your idea in post #13 is the best bet.